range (Secor & Houde 1995, Rutherford et al. 1997). High mortalities are associated with slow growth at lower temperatures, while increased predation may cause high mortality at higher temperatures (Secor & Houde 1995). Weather events, for example drops in temperature to lethal levels (12 °C), often combined with wind events that disrupt the retentive salt front and estuarine turbidity maximum (ETM), can generate high, cohort‐specific mortalities (Secor et al. 1995, Rutherford et al. 1997).
Growth and survival of Morone saxatilis larvae are primarily density independent (Kimmerer et al. 2000, Martino & Houde 2012) and responsive to the sufficiency of zooplankton prey resources and the timing of prey availability in nursery areas (i.e. supporting the match‐mismatch hypothesis; Cushing 1990). Timing of production of two key prey, the copepod Eurytemora carolleeae (= affinis) and a cladoceran Bosmina sp., is recognised as important for production of M. saxatilis larvae (Limburg & Pace 1999, Campfield & Houde 2011, Vanalderweireldt et al. 2019a).
The salt front and estuarine turbidity maximum regions of estuaries, which are important to support growth and survival of Morone saxatilis larvae, are best developed under conditions of moderate‐to‐high freshwater discharge that favour retention of eggs and larvae (Kimmerer et al. 2001, North & Houde 2001, 2003, Martino & Houde 2010). The positive relationship between spring or spring‐summer, freshwater discharge and survival of larvae, and subsequent juvenile production, is documented for Chesapeake Bay (Martino and Houde 2004) (Figure 3.21). Retention and up‐estuary transport of larvae near the salt front and ETM were demonstrated in larval mark‐recapture experiments (Secor et al. 1995, 2017) in which millions of chemically marked, hatchery‐produced larvae were released into two Chesapeake Bay tidal tributaries.
Figure 3.21 Relationship between freshwater flow in spring and summer months and young‐of‐the‐year juvenile recruitment levels (mean catch per seine haul) for Morone saxatilis in Chesapeake Bay. Years 2001 (average flow), 2002 (low flow) and 2003 (high flow) are indicated
(from Houde (2016, his Figure 3.23), modified from figure 5 in Martino & Houde (2004).
Data from Maryland Department of Natural Resources (https://dnr.maryland.gov/fisheries/pages/striped‐bass/juvenile‐index.aspx).
3.6.6 Gadidae and Clupeidae (Baltic Sea)
The Baltic Sea is a large enclosed, saline water body that supports reproduction by marine and freshwater fishes. For the gadid Gadus morhua, a typically marine species, the ambient salinity in the Baltic Sea is insufficient to maintain floating eggs and they sink to a depth of neutral buoyancy such that peak abundance occurs near the halocline in the Bornholm Basin, with smaller numbers in the more saline deep layer (Westin & Nissling 1991, Nissling et al. 1994, MacKenzie et al. 1996, Wieland & Jarre‐Teichmann 1997). Larvae of G. morhua typically hatch within 15 days of spawning and migrate vertically through the halocline into the low‐salinity surface layers (30–40 m depths) to feed (Grønkjær & Wieland 1997, Grønkjær et al. 1997). Dispersal of G. morhua larvae is primarily resulting from wind‐driven circulation in the Baltic Sea (Voss et al. 1999). Wind stress results in Ekman transport within coastal jets along both coasts of the Bornholm Basin. Vertical distributions of the larvae indicate that drift in the Bornholm Basin mainly occurs in a compensating return flow below the Ekman layer (Hinrichsen et al. 2001, 2003). Thus, periods of low wind, especially from northern and eastern directions, retain early‐life stages of G. morhua within the deepwater region of the Bornholm Basin (Hinrichsen et al. 2001). Upon transition to the juvenile stage, most juveniles inhabit deeper sites close to the Bornholm Basin.
Reproductive and recruitment success of the eastern Baltic Gadus morhua has declined in response to changing climate conditions that have reduced salinity and dissolved oxygen on the spawning grounds. These conditions, combined with high fishing pressure on adults and probable high egg predation by the clupeid Sprattus sprattus, drove G. morhua recruitment to low levels in the early 1990s (Westin & Nissling 1991, Wieland & Jarre‐Teichmann 1997). Low recruitment persisted, despite improving hydrographic conditions for egg survival in the mid‐1990s, due to insufficient larval prey concentrations, i.e. low abundance of the copepod Pseudocalanus sp. (Köster et al. 2005).
Adults of Clupea harengus migrate from offshore in the Baltic Sea to the coast where they spawn demersal eggs (Parmanne et al. 1994, Arrhenius & Hansson 1996). Early‐stage larvae reside in the littoral zone (Urho & Hilden 1990) until 30 mm length, after which they migrate to the offshore pelagic zone. Year‐class strength of western Baltic C. harengus is determined early in life (Oeberst et al. 2009). Factors affecting survival of larvae >20 mm in length and juveniles are similar amongst years, suggesting that inter‐annual variability in mortality of the late‐stage larvae and juveniles is relatively small. Unusually high mean surface water temperature during summer may reduce abundance of C. harengus larvae (Arula et al. 2016). Similarly, in the western Baltic, Dodson et al. (2019) concluded that adult C. harengus may shift dates of spawning to partly mitigate effects of variable rates of spring warming that compromise successful hatching. In that study, the abundance of preflexion and flexion C. harengus larvae was defined by dome‐shaped responses to temperature, with maximum abundances observed in the 10.3–13.4 °C (preflexion) and 13.7–18.5 °C (flexion) ranges.
In recent decades, year classes of Clupea harengus have become more abundant, apparently responding favourably to prevailing weather conditions, including mild winters in the Baltic region, with above normal rainfall leading to increased river run‐off and reduced frequency of major, high‐salinity inflows from the North Sea (Matthäus & Schinke 1994, Ojaveer et al. 2011, Arula et al. 2016, ICES 2018). The favourable conditions for reproduction have resulted in a doubling of biomass in the Gulf of Riga in recent years (Arula et al. 2014, 2016).
Another abundant clupeid, Sprattus sprattus, spawns pelagic eggs and its larvae predominantly occur in surface waters of the Baltic Sea (Voss et al. 2003) where variable, wind‐driven circulation patterns result in dispersal of early‐life stages. A retention index based on hydrodynamic modelling (Baumann et al. 2004) for the Bornholm Basin, a major S. sprattus spawning area (Köster et al. 2001) is significantly related to abundance of age‐0 S. sprattus recruits in the central Baltic. Retention within the deep‐basin benefits recruitment, while dispersion to the southeastern Baltic leads to lower abundances and recruitment failure (Baumann et al. 2006, Voss et al. 2012). Recruitment success of young‐of‐the‐year S. sprattus in the Baltic Sea is positively related to spring‐summer temperatures for which statistical models have predictive power (MacKenzie et al. 2008). Bottom‐up trophic processes apparently play a more important role in controlling recruitment of Baltic S. sprattus than do top‐down predation processes, although numerous factors including feeding, predation and water circulation patterns during the larval and also the juvenile stages contribute to recruitment variability (Voss et al. 2012). Adult spawning biomass may be less important as a regulator of year‐class size (Figure 3.22).
Figure 3.22 Recruitment of Sprattus sprattus in the Baltic Sea showing the fates of the 2002 and 2003 year